libxl: Further fix exit paths from libxl_device_events_handler

libxl: Further fix exit paths from libxl_device_events_handler

On the success path, do not call GC_FREE explicitly.  Instead, call
AO_INPROGRESS.

GC_FREE will free the gc underlying the long-term ao, which is then
subsequently referenced in backend_watch_callback's call to
libxl__nested_ao_create.  It is a miracle that this ever works at all.

Also, add an `if (rc) goto out;' after the xswatch registration.

After this, libxl_device_events_handler has the conventional and
correct ao initiation pattern.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Roger Pau Monne <roger.pau@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>